1. Field of the Invention
The present invention generally relates to a tire pressure detector, and in particular to tire pressure detectors employing a reduced power consumption mechanism.
2. Related Art
In a vehicle, a tire pressure detector may be used in conjunction with a Tire Pressure Monitoring System (TPMS) to measure air pressure in tires. Tires having low tire pressure may raise safety concerns. The tire pressure detector may be attached to a tire, or the wheel on which the tire is mounted, and may detect low tire pressure. Upon detection, the tire pressure detector may transmit this information to a driver via a TPMS or the like. The transmission of the tire pressure information is often performed wirelessly. For instance, a radio frequency transmission may be used to transmit tire pressure detector readings.
In a tire pressure detector, sensors in addition to a pressure sensor such as a temperature sensor and/or a speed sensor may be included. A battery is also typically used to power components of the tire pressure detector. The tire pressure detector may also include communication components such as a phase locked loop (PLL) circuit, a crystal oscillator, an RF antenna that sends radio transmissions to a central control unit and possibly receives radio transmissions, etc. A power amplifier may also be included such that tire pressure signals may be amplified for transmission.
In a conventional tire pressure detector, the PLL circuit and the power amplifier are often installed on a single integrated circuit (IC) chip. FIG. 1 is a block diagram of a conventional PLL circuit 10. The conventional PLL circuit 10 typically includes a phase detector 20, a voltage controlled oscillator (VCO) 30 and an amplifier 40, as one of ordinary skill in the art will appreciate. The PLL circuit 10 may be constructed as a single integrated circuit. Although not shown in FIG. 1, the power amplifier is typically disposed on the same IC chip that defines the PLL circuit 10.
The conventional PLL circuit 10 operates as follows. The phase detector 20 compares two input frequencies, “fin” and “fvco”, and generates output that corresponds to a phase difference of the two input frequencies. If “fin” does not equal to “fvco”, a phase-error signal is generated. The phase-error signal may be filtered by a suitable filter 50 and amplified by the amplifier 40. In response to the phase-error signal, “fvco” is deviated in the direction of “fin”. If the “fin” does equal to “fvco”, the VCO 30 will lock to “fin”. The phases of the two input frequencies, “fin” and “fvco” become in phase.
In the conventional PLL circuit 10, the VCO 30 is always powered-on regardless of logic states of input data bits. Even if input data bits have a zero state, the VCO 30 remains on and consumes current. Further, various components on the IC chip may not be separately controllable. The PLL circuit 10 and a power amplifier formed on the single IC chip may also generate noise.